M. A. J. Angstrom's Optical Researches. 341 



the equations of motion will be satisfied by the following assump- 

 tion : — 



£=e -fa' cos (nt + X) +acos2 (nt + X) 

 v = e ' + b' cos (nt + X) + f3 cos2 {nt + X) 

 £= e" + d cos (nt + \) + 7 cos 2 (nt + X), 



so that a simple augmentation of the amplitudes can excite 

 vibrations of a higher order, which in their turn can produce 

 chemical effects. 



22. A question of great interest, to the solution of which, 

 however, great difficulties oppose themselves, is the following : — 

 Why is it tbat the spectrum obtained from the glowing metallic 

 particles of the electric spark exhibits only certain colours, and 

 does not, like the glowing metal itself, show all colours in a con- 

 tinuous series? A glowing platinum wire gives a spectrum 

 without shining lines ; the electric spark from platinum gives 

 such lines in numbers. Not without some hesitation as to 

 whether I have hit upon the true cause of this difference, do I 

 venture upon the following explanation. 



The general theory of small motions teaches us that the number 

 of the molecular motions of a body amounts to thrice the num- 

 ber of molecules within the sphere of attraction ; and from this 

 it follows, that when the number of the molecules is infinite the 

 number of their motions must also be infinite, and hence the 

 spectrum must exhibit itself as a continuous whole. This, 

 however, is not necessarily the case with the metallic particles 

 which are scattered by the electricity. These particles may, in 

 magnitude, remain considerably under the sphere of attraction 

 of the molecules ; by this the number of the possible oscillations 

 becomes limited, and hence also the spectrum cannot form a 

 connected whole. 



23. I have had only recently an opportunity of becoming 

 fully acquainted with the interesting memoir of Stokes " On tbe 

 Change of Refrangibility of Light." I see with satisfaction that 

 Stokes's explanation of the remarkable phenomena of dispersion 

 in the green colours of plants, in sulphate of quinine, and in an 

 infusion of horse-chestnut bark, namely, that the medium, when 

 illuminated by the sun, becomes itself luminous, is exactly the 

 same as that which I have given in the foregoing pages of the 

 same phenomenon. 



24. The law at which Stokes has arrived is, that in the case 

 of inner dispersion, the refrangibility of the incident light is 

 changed, being generally diminished; or, what is the same, that 

 the time of oscillation of the dispersed light is, in comparison 

 with the incident light, augmented. This law is by no means 

 Opposed to dynamical principles. The unchangeability of the 



